In the present study, the relationship between the sequential change in oxygenation and that in the various aspects of vascular function following X-irradiation will be investigated in Walker Carcinoma 256 of rats and SCL mammary tumor of mice. Specifically, the functional intravascular volume and vascular permeability will be measured simultaneously by Cr51-RBC and I125-plasma protein respectively. The rate of blood flow will be quantitated using the clearance rate of Xe133 from tumor after intestitial injection. The vascular occlusion in the irradiated tumors will be investigated by autoradiographic study of the distribution of Cr51-RBC in cappillaries as well as the distribution of i.v. injected lead oxide and silver nitrate. Histlpathologic changes of vascular patterns will be investigated using Bielschowsky's silver stain. The proportion of hypoxic cells in the tumors before and after irradiation will be computed from the cell survival curves.